Normal human peripheral blood mononuclear cells cultured in vitro can be doubly infected with human immunodeficiency virus (HIV) and cytomegalovirus (CMV). Doubly infected cells produce augmented titers of HIV. In contrast to CMV, neither herpes simplex virus, type 1 or 2, augmented HIV replication in doubly infected cultures of H9 cells. The capacity of various T and B cells lines to support replication of HIV, simian immunodeficiency virus (SIV) and SAIDS retrovirus type 1 (SRV-1) was studied. We have found that the T-cell lines, CEM, Rex, Molt-3 and Peer, all of which contain both CD4 and CD8 surface markers, supported HIV replication. SIV replicated in only two of the T-cell lines, CEM and Molt-3. A T-cell line, HSB-2, which had neither CD4 or CD8 surface markers, did not support HIV, but the titers of infectious virus produced were quite low. SRV-1 did not replicate in any of the T-cell lines studied, but replicated well in B-cell lines containing the EBV genome. We have isolated a retrovirus with characteristics of SIV from three wild-caught, African green monkeys. Rhesus monkeys infected with these SIV isolates have developed an immunosuppressive disease similar to human AIDS. An epizootic of simian acquired immunodeficiency syndrome (SAIDS), caused by a Mason-Pfizer-like type D retrovirus, has been detected in rhesus (M. mulatta) and cynomologous monkeys (M. fasicularis) housed in the NIH colony. Neurtralization tests have shown this Mason-Pfizer-like virus to be SAIDS retrovirus, type 2 (SRV-2). Procedures have been formulated to prevent further spread of this agent to susceptible animals.